Our earlier results of immunotitration studies had suggested that acetyl CoA carboxylase isolated from transplantable mammary tumors (R3230 AC and 13762) was different from the enzyme obtained from rat lactating mammary gland. This conclusion was based upon the observations that neutralization of carboxylase activity present in the cellular extracts of tumors required much higher amounts of anti-acetyl CoA carboxylase than was needed to inhibit an equivalent amount of the carboxylase activity of normal tissue origin. Recently a number of laboratories have provided evidence suggesting that acetyl CoA carboxylase may be regulated by phosphorylation/dephosphorylation mechanism. The dephospho form of the enzyme is believed to be active whereas the phospho form is less active. In view of these observations, we want to determine whether the carboxylase of neoplastic origin mainly exists in a modified form which is enzymatically less active. Similary, we are attempting to isolate and characterize the converter enzymes from rat mammary gland that add and subtract phosphoryl group(s) from the highly purified acetyl CoA carboxylase. We are also continuing our studies on the mechanism of inhibition of fatty acid synthetase by S-(-4-bromo-2,3-dioxobutyl)-CoA.